Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor storage canister, and then purge the stored vapors during a subsequent engine operation. Specifically, purging of the stored vapors during engine operation may include commanding open a canister purge valve positioned in a purge line between the engine, and commanding open a canister vent valve positioned in a vent line that couples the canister to atmosphere. In this way, engine intake manifold vacuum may be applied to the canister whereby atmospheric air may be drawn across the canister, the atmospheric air desorbing the stored fuel vapors from the canister and routing them to the engine for combustion.
However, certain vehicles may face challenges in effectively purging adsorbed fuel vapor from a fuel vapor storage canister. As one example, hybrid electric vehicles and/or vehicles equipped with start/stop capability may encounter limited engine run time in which to purge the canister. Further issues may be encountered for vehicles designed to reduce engine intake manifold vacuum, as engine intake manifold vacuum is a pumping loss. For example, vehicles with engines equipped with twin independent variable cam timing (TiVCT) may operate with low engine intake manifold vacuum, which may decrease opportunities for effectively purging the canister. In yet another example, boosted engines may often operate under conditions of positive engine intake manifold vacuum, thus decreasing opportunities for purging the canister. While such boosted engines may have specific hardware such as check valves and an ejector for facilitating purging during boosted operation, such purging may not be efficient, as vacuum generated in such engines equipped with an ejector may be limited due by choke flow. Thus, it is desirable for such vehicles to take advantage of any opportunity to purge the canister, in order to reduce opportunities for bleed emissions from the canister to atmosphere, which may occur if the canister is not otherwise frequently purged.
Toward this end, U.S. Pat. No. 9,739,248 teaches that during engine-off conditions when ambient temperatures are decreasing, vacuum in a fuel tank may increase to a point such that a vacuum-based valve is opened, drawing air through the fuel vapor canister thereby purging the canister and delivering fuel vapor back to the tank. Such purging of the canister is referred to as a passive purge, as opposed to an active purge that relies on engine intake manifold vacuum. However, the inventors herein have identified several issues with such an approach. First, depending on ambient conditions (e.g. temperature, precipitation amounts, level of wind, etc.), sufficient vacuum may not occur to enable passive purging of the canister. Second, such a method relies on a specifically designed valve (e.g. vacuum-based valve), which may not be desirable to include in all vehicle fuel systems and which may increase costs associated with vehicle assembly. Third, such a vacuum-based valve may only have one set point at which pressure in the fuel system is reached, thus limiting an ability to exert any level of control over how much a canister may be passively purged. Fourth, such a method relies on the vehicle soaking with the engine off for significant periods of time. As more and more vehicles participate in car-sharing models where a vehicle may be rented for short periods of time, such extended vehicle-off soak periods may be very infrequent, and thus, opportunities for passive purging of the canister may too be infrequent.
The inventors herein have recognized the above-mentioned issues, and have developed systems and methods to at least partially address them. In one example, a method comprises sealing a fuel system of a vehicle and then descending an altitude change that is predicted in advance of the vehicle descending the altitude change, and subsequent to the vehicle descending the altitude change, unsealing the fuel system to passively purge fuel vapors stored in a fuel vapor storage canister to a fuel tank of the vehicle. In this way, a loading state of the canister may be reduced during vehicle operation without relying on engine operation. In one example, the altitude change predicted in advance may comprise an altitude change that occurs along a learned driving route, where the learned driving route is learned over time. In another example, the altitude change known in advance may be based on a vehicle operator-selected or passenger-selected driving route.
In some examples, the altitude change that is predicted in advance may result in development of a negative pressure in the fuel system with respect to atmospheric pressure that is at least a predetermined negative pressure, for example −8 InH2O.
In yet another example, such a method may comprise unsealing the fuel system to passively purge fuel vapors stored in the fuel vapor storage canister to the fuel tank while the vehicle is descending the altitude change, under conditions where a predetermined passive purge threshold negative pressure is reached in the fuel system during the descending the altitude change, and resealing the fuel system in response to pressure in the fuel system being within a threshold of atmospheric pressure (e.g. not different from atmospheric pressure by more than 5%) during the descending the altitude change. In some examples, the predetermined passive purge threshold negative pressure may comprise −16 InH2O. Furthermore, unsealing the fuel system to passively purge fuel vapors stored in the fuel vapor storage canister to the fuel tank while the vehicle is descending the altitude change, and resealing the fuel system in response to the fuel system being within the threshold of atmospheric pressure, may occur any number of times during the time the vehicle is descending the altitude change.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.